A carbon nanotube is a new substance discovered by Sumio Iijima in 1991, which can exhibit a metallic and/or semiconducting property depending on the diameter thereof and the way the tube is wound. The individual physical property of the carbon nanotube is entirely different depending on the structure of the tube, and study within the art is currently being vigorously undertaken. In addition, the carbon nanotube is a substance, of which much is expected as a next generation material for use in devices and the like, having applications in the field of electronics and energy.
From studies on a process for producing a single-walled carbon nanotube (SWNT), an industrially low-cost mass-production of the carbon nanotube (the Chemical Vapor Deposition or CVD method) has nearly been established, for example, by decomposition of a hydrocarbon using ferrocene as a catalyst (see, for example, Non-Patent Document 1). As such, the carbon nanotube has been commercialized. As a representative example of a method for synthesizing a single-walled carbon nanotube, there are an arc discharge method and a laser evaporation method (e.g., see Non-Patent Document 2). The carbon nanotube is further purified by ultrafiltration (e.g., see Non-Patent Document 2), wherein a purity of 90% or more is obtainable.
The distribution of the diameters of single-walled carbon nanotubes produced by an arc discharge differs depending on the type of metal catalyst to be used in the synthesis. In this way, it is possible to control the average diameter of the carbon nanotube by selecting the type of metal catalyst, and thereby the distribution of the diameter of the carbon nanotube produced can be controlled with an average diameter in the range of ±0.4 nm. However, any method among the existing production methods does not allow a selective synthesis of a single-walled carbon nanotube which has a particular diameter.
Therefore, studies for establishing a method for the separation and purification of certain carbon nanotubes from carbon nanotubes given by the above-mentioned existing production method, have been carried out in order to investigate the characteristic physical properties of said individual carbon nanotubes separated and purified.
For an example, P. Umek and D. Mihailovic carried out agarose gel electrophoresis of single-walled carbon nanotubes dispersed in aqueous sodium dodecyl sulfate (SDS) solution, followed by hydrochloric acid treatment, removal of SDS using deionized water, desiccation, and Raman spectroscopy examination of the resultant respective fractions. This confirmed that the single-walled carbon nanotubes were partially separated on the basis of diameter and length thereof (see, e.g., Non-Patent Document 3).
Further, Stephen K. Doom et al. carried out capillary electrophoresis of a solution of carbon nanotubes dispersed in SDS and found, from absorption spectra and Raman spectra of respective separated carbon nanotubes, that the single-walled carbon nanotubes could be separated depending on differences in the elution time among respective carbon nanotubes, which reflects the difference in the length thereof. (see, e.g., Non-Patent Document 4). The above-mentioned studies have nearly established a method for separating the carbon nanotube based on the length thereof.
However, the characteristic physical property of the carbon nanotube is determined depending on a multiple physical properties such as the diameter and the chiral angle thereof, which means that separation of carbon nanotubes based only on the length thereof does not necessarily correspond to the separation based on the characteristic physical property thereof. Therefore, to date, the length-based separations of carbon nanotubes is not sufficient to define the characteristic physical property thereof.
Although many studies have been performed so far on carbon nanotubes, the precision has still remained very low for the preparation, separation or purification of single-walled tubes which have the same diameter, chirality, work function, and band gap (see, e.g., Non-Patent Documents 5 to 12). With respect to the resultant separation based on diameter, Non-Patent Document 9, which discloses a separation of DNA-CNT by ion exchange chromatography, is an example of a related prior art. However, it is completely different in the principle from the present invention, and inferior to the technique of the present invention in separation precision. Further, there are some patent applications directed to a method for purification of a carbon nanotube (see, e.g., Patent Documents 1 to 5). However, all of these Patent Documents 1 to 5 only disclose techniques of impurity removal. Thus these documents do not describe any separation of carbon nanotubes which have a uniform characteristic physical property, wherein diameter, chiral angle and the like thereof, are respectively the same.
Although various studies have contributed to the characterization of the electron structure of carbon nanotubes, which are dependent on the structures of respective carbon nanotubes, very limited information is available on the absolute potential in the energy level of the carbon nanotube, with the widespread impression that while a monomolecular carbon nanotube has various structures, the absolute potential of the Fermi level of individual carbon nanotubes is considered to be at a similar level. By investigating spectral features of Raman scattering (especially in the radial breathing mode (w=150-240 cm−1)) of isolated single-walled carbon nanotubes (SWNT) which are metallic or semiconducting in solution under a potential control, we are the first in the world to have discovered that the Fermi level of tubes was found to positively shift greatly with the decrease of tube diameters. These observations suggest that the work function of the tube depends heavily upon the structure of the SWNT. Further, we also have discovered that the structural dependence of a metallic carbon nanotube is significantly larger than that of a semiconducting carbon nanotube. The great difference in the work function means that, for example, a carbon nanotube with a specific diameter is more stable than a noble metal (e.g., Au and Pt), and that on the other hand, a carbon nanotube with a larger diameter has the same degree of tendency to release electrons as Mg and Al. Based on the above discussions, it has first been made clear that the characteristic physical properties of a single carbon nanotube has a significant dependence upon the diameter, chirality thereof, and the like.
In order to put carbon nanotubes, which are expected to be the next generation material, into practical utilization, it is inevitably required to control the physical properties dominated by the diameter and chirality thereof, and the like. Therefore, an innovative separation method has to be established to sort the carbon nanotube in accordance with the desired physical properties to be utilized.    Patent Document 1: Japanese Patent Application Laid-Open No. 8-198611    Patent Document 2: JAPANESE PATENT APPLICATION LAID-OPEN No. 2003-81616    Patent Document 3: JAPANESE PATENT APPLICATION LAID-OPEN No. 2003-300714    Patent Document 4: JAPANESE PATENT APPLICATION LAID-OPEN No. 2003-212526    Patent Document 5: JAPANESE PATENT APPLICATION LAID-OPEN No. 2002-515847    Non-Patent Document 1: Kazuyoshi Tanaka, Challenge to Carbon Nanotube Device, Kagaku Dojin (2001)    Non-Patent Document 2: Yahachi Saito and Shunji bando, Basis of Carbon Nanotube, Corona (1998)    Non-Patent Document 3: P. Umek and Mihailovic, Synthetic Metals, 121, 1211-1212 (2001)    Non-Patent Document 4: Stephen K. Doom, Robert E. Fields, III, Hui, Hu, Mark A. Hamon, Robert C. Haddon, John P. Selegue, and Vahid Majidi, J. Am. Chem. Soc., 124, 3169-3174 (2002)    Non-Patent Document 5: R. Kfupke et al., Science, 301, 344-347 (2003)    Non-Patent Document 6: G. S. Duesberg et al., Chem. Comm., 435-436 (1998)    Non-Patent Document 7: G. S. Duesberg et al., Appl. Phys., A67, 117-119 (1998)    Non-Patent Document 8: D. Chattopadhyay et al., J. Am. Chem. Soc., 124 728-729 (2002)    Non-Patent Document 9: M. Zheng et al., Science, 302, 1545-1548 (2003)    Non-Patent Document 10: H. Dodziuk et al., Chem. Comm., 986-987 (2003)    Non-Patent Document 11: D. Chattopadhyay et al., J. Am. Chem. Soc., 125, 3370-3375 (2003)    Non-Patent Document 12: Z. H. Chen et al., Nano Lett., 1245-1249 (2003)